Electrospray is a method of coupling ions derived from a liquid source such as a liquid chromatograph or capillary electrophoresis system into a vacuum analysis system such as a mass spectrometer (Whitehouse et al. 1985; U.S. Pat. No. 4,531,056). The liquid is typically a dilute solution of analyte in a solvent. The spray is induced by the action of a strong electric field at the end of capillary containing the liquid. The electric field draws the liquid out from the capillary into a Taylor cone, which emits a high-velocity spray at a threshold field that depends on the physical properties of the liquid (such as its conductivity and surface tension) and the diameter of the capillary. Increasingly, small capillaries known as nanospray capillaries are used to reduce the threshold electric field and the volume of spray (U.S. Pat. No. 5,788,166).
The spray typically contains a mixture of ions and droplets, which in turn contain a considerable fraction of low-mass solvent. The problem is generally to couple the majority of the analyte as ions into the vacuum system, at thermal velocities, without contaminating the inlet or introducing an excess background of solvent ions or neutrals. The vacuum interface carries out this function. Capillaries or apertured diaphragms can restrict the overall flow into the vacuum system. Conical apertured diaphragms, often known as molecular separators or skimmers can provide momentum separation of ions from light molecules from within a gas jet emerging into an intermediate vacuum (Bruins 1987; Duffin 1992; U.S. Pat. No. 3,803,811, U.S. Pat. No. 6,703,610; U.S. Pat. No. 7,098,452). Off-axis spray (USRE35413E) and obstructions (U.S. Pat. No. 6,248,999) can reduce line-of-sight contamination by droplets, and orthogonal ion sampling (U.S. Pat. No. 6,797,946) can reduce contamination still further. Arrays of small, closely spaced apertures can improve the coupling of ions over neutrals (U.S. Pat. No. 6,818,889). Co-operating electrodes (U.S. Pat. No. 5,157,260) and quadrupole ion guides (U.S. Pat. No. 4,963,736) can apply fields to encourage the preferential transmission of ions. The use of a differentially pumped chamber containing a gas at intermediate pressure can thermalise ion velocities, while the use of heated ion channels (U.S. Pat. No. 5,304,798) can encourage droplet desolvation. The device of U.S. Pat. No. 5,304,798 is fabricated in a thermally and electrically conductive material, and is a massive device, the heated channel being of the order of 1-4 cm long.
Vacuum interfaces are now highly developed, and can provide extremely low-noise ion sampling with low contamination. However, the use of macroscopic components results in orifices and chambers that are unnecessary large for nanospray emitters and that require large, high capacity pumps. Furthermore, the assemblies must be constructed from precisely machined metal elements separated by insulating, vacuum-tight seals. Consequently, they are complex and expensive, and require significant cleaning and maintenance.